The present invention relates generally to current sensing devices for electrical systems, and more particularly to current reset fault indicators and magnetic core assemblies for use therein.
Detection of fault currents in power distribution cables, such as those used in underground electrical distribution systems, is often most readily accomplished by means of automatically-reset clamp-on type fault indicators, which are attached to the cable being monitored and include a reed switch or other magnetically-actuated means in magnetic communication with the monitored conductor to condition an indicator to a fault-indicating state upon the occurrence of a fault current, and a reset circuit for returning the indicator to a reset-indicating condition upon restoration of line current. Examples of automatically reset fault indicators are found in U.S. Pat. Nos. 3,676,740, 3,906,477, 4,086,529, and 4,234,847 of the present inventor.
Automatically-reset clamp-on type fault indicators typically derive their operating power directly from current flow in the monitored conductor. In such current-reset fault indicators a magnetic core assembly is generally required to provide a concentrated magnetic flux from current flow through the conductor. A magnetic winding in magnetic association with the core assembly converts the concentrated flux to an electric current which is rectified for use in powering the fault indicator.
For optimum flux pick-up it is desirable that the magnetic core assembly be in the form of a closed loop which completely encircles the conductor. However, to enable the core assembly to be installed and removed from the conductor, it is necessary that connection means be provided by which the loop can be opened. One form of magnetic core assembly which has proved particularly successful in this regard is described in U.S. Pat. No. 3,725,832, which issued to the present inventor on Apr. 3, 1973. In this construction a circular magnetic core is provided which comprises a plurality of laminations of pre-stressed oriented silicon steel secured together near their juxtaposed ends so as to form an annular loop around a monitored conductor. The ends are provided with interlocking tongue-and-groove type members formed of corrosion resistant magnetic material which allow the loop to be opened for installation and removal on a conductor. The present invention constitutes an improvement in this form of assembly.
One requirement of closed loop magnetic core current-reset fault indicators having magnetic sensing means such as a reed switch or magnetic sensing coil for fault detection is that adequate coupling be maintained between the magnetic sensing means and the monitored conductor when the fault indicator is installed on cables of different diameters. Because of safety considerations it is usually not practical to tie or otherwise secure smaller diameter cables to the core assembly so as to deliberately position the cables adjacent the magnetic reed switch. The present invention obviates this problem by providing a construction whereby the monitored cable is always maintained in close proximity to the magnetic sensing means of the fault indicator, notwithstanding variations in cable diameter.
A further drawback of prior art closed loop magnetic core assemblies has been their susceptibility to being inadvertently pulled off the monitored conductor, as when force is exerted on leads connected to the circuit module. The present invention overcomes this problem by providing a positive spring-biased locking engagement in the closed loop core assembly whereby forces exerted on connecting leads only serve to more securely lock the core assembly in place and removal is possible only by deliberate manipulation of the core assembly by the use of lineman's tools or other appropriate means.
Accordingly, it is a general object of the present invention to provide a new and improved current-reset clamp-on type fault indicator.
It is a more specific object of the present invention to provide a new and improved current-reset clamp-on type fault indicator having an improved magnetic core assembly.
It is another object of the present invention to provide a clamp-on type fault indicator which is readily adaptable to a wide range of cable diameters.
It is another object of the present invention to provide a clamp-on type fault indicator which is less susceptible to being inadvertently pulled free of the monitored cable.
It is another general object of the present invention to provide a new and improved magnetic core assembly for use in fault indicators and the like.